1. Field of the Invention
This invention relates generally to light source devices with arrayed light emitting elements, and more particularly to a light source device with arrayed light emitting elements, employed as a light source for reading image information in an adhesion type image sensor, an optical image reader, a contraction type image sensor or the like, and a manufacturing method thereof.
2. Description of the Background Art
In reading image information, in optical image readers, facsimile devices, copiers, various kinds of optical checking devices and the like, light from a light source is directed onto a surface o a document or that of an object to be checked, and the reflected light is then subject to photoelectric conversion by employing an adhesion type image sensor, a linear image sensor, a CCD or the like to be inputted as information. A light emitting diode element (hereinafter referred to as LED) which is superior in reliability and life has been in wider use as the light source.
Since a simple LED emits a small quantity of light and thus is only capable of local illumination, a light emitting element array (hereinafter referred to as an LED array) including a plurality of LEDs linearly arranged is commonly used.
The structure of a conventional LED array employing LED chips is shown in FIGS. 1A-1C. Referring to these figures, the conventional LED array comprises a plurality of LED chips 2 directly die-bonded onto and linearly arranged in an array on a printed circuit board 1.
The LED chips 2 are arranged a predetermined spacing apart from each other in order to satisfy any requirements in design. In addition, reflection plates 3 are provided between any LED chips inside in order to enhance homogeneity of light directed onto a surface of illumination and also to reduce the total number of the LED chips 2. The reflection plates 3 are made such as of white resin.
A convergent lens 4 in the form of a rod, provided adjacent and parallel to the array of the LED chips 2, is firmly secured by a reflection casing 5 and holders 6 for supporting opposite end faces of the convergent lens 4. This convergent lens 4 serves to converge luminous flux 7 exiting the array of the LED chips 2 into a line within a predetermined range of the illumination surface 8. The convergent lens 4 is made such as of transparent acrylic resin.
Since the luminous flux of the LED array is composed of direct light from the LED chips 2 and indirect light from reflection on the reflection plates 3, the reflection casing 5 or the like, the light converged at the convergent lens 4 is not completely converged on the surface of the document placed on the illumination surface 8. Consequently, such a light ray exists as to be in focus even at a position considerably distant from the surface of the document. This configuration ensures the steady quantity of light required for reading the document even when the surface of the document is curved or when there is a slight error in accuracy in positioning of an optical system.
FIG. 2 shows one example of an optical image reader employing an LED array as a light source. With reference to this figure, the luminous flux 7 which exits the LED chip 2 on the printed plate board 1 is converged at the illumination surface 8 of the document. The entire LED array as a light source is held by a fixing jig 9. The reflected light from the document is again converged at a selfoc lens 10 and then enters a line image sensor 11, in which the information is photoelectrically converted into an electrical signal.
FIG. 3 shows a cross sectional structure of an adhesion type image sensor employing the LED array as a light source. Referring to this figure, in the adhesion type image sensor, light from an LED array 12 used as a light source first illuminates a document surface 14 through an optical fiber array 13 coated with light absorbing material. The light scattered on the document surface 14 is propagated through the optical fiber array 13 and through an optical fiber array 15 coated with light absorbing material and is then introduced into a photodetector 16. The photodetector 16 is shielded by a light shielding fence 23 from direct light from the LED array 12. Image information detected by this, photodetector 16 is processed by an LSI circuit 18 on a base board 17.
The LED array used as a light source is also employed as a light source for a conventional contraction type image sensor employing a contraction optical system. FIGS. 4A and 4B schematically show a comparison between a schematic configuration of the optical system of the contraction type image sensor and that of the adhesion type image sensor shown in FIG. 3. Referring to these figures, in the contraction type image sensor, light emitted from a light source 19 is reflected by a document 20, contracted by an optical lens 21 and then imaged onto a CCD sensor 22. Meanwhile, in the adhesion type image sensor, light which is emitted from the light source 19 and then reflected at the document 20 travels in parallel through an optical fiber array 15 and is then directed onto a photodetector 16 to the same scale as that of image information on the document 20.
During assembly of the conventional LED array used as a light source, as shown in FIGS. 1A-1C, it is required to integrally assemble the printed plate board 1 comprising the LED chips 2 attached thereto, the reflection plates 3, the reflection casing 5, the holders 6 and the convergent rod lens 4 and allow the array of the LED chips 2 and the convergent lens 4 to optically align with each other, thereby preventing an occurrence of an error in relation to relative positioning of these parts. In order to fulfill this requirement, a process for assembling the LED array used as a light source requires a considerably long assembly time and a highly accurate operation.
As described above, the parts are required to be assembled with high accuracy regarding the manufacture of the conventional LED array used as a light source, thereby preventing reductions in cost, size and weight.
Furthermore, in the conventional LED array used as a light source, a bonding area on the printed plate board 1 is required to be plated with noble metal such as gold for direct bonding of the LED chips onto the printed plate board 1. This noble metal plating is necessary because the LED chips 2 are liable to be contaminated with copper ions and thus need to avoid any contact with copper. For this reason, the cost of the LED array has become remarkably increased.
Moreover, the conventional LED array used as a light source has a disadvantage that total reflection at the interface of the LED chips 2 and air-gap, provided between the LED chips 2 and the convergent lens 4, causes a decrease in external quantum efficiency of the LED array.